Fiber material for reinforcing plastics

ABSTRACT

A fiber material for reinforcing plastics prepared by laminating at least one first fiber substrate in which the reinforcing fibers extend in two directions including the longitudinal direction and the transverse direction intersecting therewith at a substantially right angle with at least one second fiber substrate in which the reinforcing fibers extend in two directions including directions having angles of ±(25-65) degree relative to the longitudinal direction. Or, a fiber material for reinforcing plastics prepared by laminating at least one first fiber substrate in which the reinforcing fibers extend in at least one of two directions including the longitudinal direction and the transverse direction intersecting therewith at a substantially right angle, at least one second fiber substrate in which the reinforcing fibers extend in a direction having an angle of (25-65) degree relative to the longitudinal direction and at least one third fiber substrate in which the reinforcing fibers extend in a direction having an angle of -(25-65) degree relative to the longitudinal direction. The above laminate moldings are integrated with each other by stitch yarns passing repeatedly in the direction of lamination.

This is a continuation of application Ser. No. 410,054, filed Aug. 20,1982 now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to a reinforcing fiber material used in fiberreinforced plastics (hereinafter referred to as FRP). More particularly,this invention relates to a laminated reinforcing fiber materialsuitable for use in relatively large FRP.

As regards fiber substrates such as, for example, woven fabric or itsprepreg, which are used as reinforcing fiber materials for FRP, not sothick substrates can be obtained. Accordingly, they are usually used inthe form of a laminate consisting of a plurality of fiber substrates.

However, when such a laminate is heated under pressure to form FRP,since there is nothing to restrain the fiber substrates from movingrelative to each other, the fiber substrates or the reinforcing fibersare pushed aside by a resin flow, and the arrangement of the reinforcingfibers are disturbed. This disturbance in arrangement tends towardincreasing, particularly, in case where unidirectional prepregs in whichthe bond between the reinforcing fibers is performed by resin only isemployed or in case where a resin injection molding process consistingof resin intrusion is adopted for a material having a relatively strongtexture, such as a woven fabric.

On the other hand, when a quasiisotropic FRP is desired, fibersubstrates are laminated so that, for example, directions of theirreinforcing fibers cross at an angle of 0,±45°or 90° . When this isdone, since the reinforcing fibers and the resin have markedly differentcoefficients of linear thermal expansion, residual stresses due to thedifference in thermal strain arise between layers. Furthermore, sincethe Poisson's ratio of FRP is dependent upon the direction of thearrangement of reinforcing fibers and has a great anisotropy within thesurface, the difference in Poisson's ratio between layers, whencross-laminated, becomes considerably large. Accordingly, when a stressis given to FRP, stresses due to the above-mentioned thermal straindifference or Poisson's strain difference, in addition to an externalforce, are exerted in a complicated manner, delamination by resinrupture between layers occurs before the reinforcing fibers are broken.Especially, when high elongation carbon fibers (fibers having anelongation of about 1.7-2.2%) are used as reinforcing fibers, theabove-mentioned disadvantages become marked, because there is a greatthermal strain difference due to a marked difference between theircoefficients of linear thermal expansion (the coefficient of linearthermal expansion of carbon fiber is -(0.7-1.2)×10⁻⁶ /°C. and that ofresin is about (55-100)×10⁻⁶ /°C.), and because there is a greaterdifference in Poisson's strain due to its high elongation. Furthermore,the more fiber substrates a laminate has, that is, a thicker FRP hasmore resin layers between fiber substrates, the more marked theabove-mentioned problem becomes, causing a decrease in reliability ofFRP. Moreover, once a crack arises between layers, it propagates at astretch because there is nothing to prevent the propagation.

SUMMARY OF THE INVENTION

It is a principal object of this invention to provide a fiber materialfor reinforcing plastics, prepared by laminating a plurality of fibersubstrates and having quasiisotropy within its surface.

It is another object of this invention to provide a high-reliabilityfiber material for reinforcing plastics, in which delamination does notoccur between a plurality of fiber substrates.

It is still another object of this invention to provide a reinforcingfiber material useful for large FRP requiring high strength,particularly, such as beams.

In order to attain the above-mentioned objects, the fiber material forreinforcing plastics according to this invention is composed of at leastone first fiber substrate in which the reinforcing fibers extend in twodirections including the longitudinal direction and the transversedirection intersecting therewith at a substantially right angle and atleast one second fiber substrate in which the reinforcing fibers extendin two directions having angles of ±(25-65) degree relative to thelongitudinal direction. And said first fiber substrate and said secondfiber substrate are laminated in the same longitudinal direction, andconstructed so that they are integrated with each other by stitch yarnspassing through repeatedly in the direction of the fiber substrates inthe laminated state. And, another embodiment of a fiber material forreinforcing plastics according to this invention is composed of at leastone first fiber substrate in which the reinforcing fibers extend in atleast one of two directions including the longitudinal direction and adirection intersecting with the longitudinal direction at asubstantially right angle, at least one second fiber substrate in whichthe reinforcing fibers extend in a direction having an angle of (25-65)degree relative to the longitudinal direction and at least one thirdfiber substrate in which the reinforcing fibers extend in a directionhaving an angle of -(25-65) degree relative to the longitudinaldirection. Said respective first, second and third substrates arelaminated in the same longitudinal direction and constructed so thatthey are integrated with each other by stitch yarns passing repeatedlythrough the substrates in the direction of the fiber substrates in thelaminated state.

Still further objects of this invention will be apparent from thedescription of examples illustrated with reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view illustrating a reinforcing fiber materialaccording to an embodiment of this invention, part of the top layer ofwhich is cut away;

FIG. 2 is a perspective view illustrating the same material, wherein itsside is taken as the cross-section;

FIGS. 3 and 4 are each a plan view illustrating an embodiment showing amanner of application of stitch yarns;

FIG. 5 is a perspective view illustrating a reinforcing fiber materialaccording to another embodiment, part of which is cut away;

FIG. 6 is a perspective view illustrating a reinforcing fiber materialaccording to still another embodiment;

FIG. 7 is a perspective view illustrating a reinforcing fiber materialaccording to still another embodiment, part of which is cut away;

FIG. 8 is a perspective view illustrating a reinforcing fiber materialaccording to still another embodiment;

FIG. 9 is a perspective view illustrating a reinforcing fiber materialof this invention formed for a I-beam;

FIGS. 10A through 10I are each a perspective view illustrating anembodiment of a beam molded from a reinforcing fiber material accordingto this invention;

FIGS. 11A and 11B are plan views each illustrating a test piece formeasuring the resistance of yarn slippage of a woven fabric; and

FIG. 12 is a graph illustrating a comparison of resistance todelamination of FRP molded from a reinforcing fiber material accordingto this invention and FRP outside this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIGS. 1 and 2, the reinforcing fiber material is composed of a firstfiber substrate 1 on the top consisting of a woven fabric and a secondfiber substrate 2 on the back consisting of fabric. Of these substrates,the first fiber substrate on the top is composed of plain weave oflongitudinal reinforcing fibers 3 and transverse reinforcing fibers 4intersecting therewith, and the second fiber substrate 2 on the back iscomposed of plain weave of reinforcing fibers 5 and 6 intersecting witheach other in bias directions each having an angle of α or α' relativeto the longitudinal direction L of the fabric. The angles α and α' ofthe reinforcing fibers 5 and 6 of the second fiber substrate 2 can bevaried within the ranges: 25 to 65 degree and -25 to -65 degree,respectively, but it is generally preferred that α=45 degree and α'=-45degree.

With the two fiber substrates 1 and 2 laminated as above are engagedstitch yarns 7 at an equal pitch along the longitudinal direction of thefiber substrate. While the yarns are passing repeatedly from the top tothe back and then from the back to the top of the two fiber substrates,the yarns integrate the fiber substrates 1 and 2 on both sides. Aplurality of stitch yarns 7 are provided at intervals substantiallyequidistant in the lateral direction so that the fiber substrates 1 and2 are integrated over the entire surface. The positions of the stitchyarns 7 relative to the reinforcing fibers 3 and 4, and 5 and 6 of thefiber substrates 1 and 2 are not necessarily regular but can be random.Furthermore, as shown in FIG. 3 and FIG. 4, it is also possible to applyadditional stitch yarns 7' particularly to areas for which furtherreinforcement of integration is necessary.

The reinforcing fiber material of the above structure can givequasiisotropic physical properties to FRP in the direction within itssurface, because the first fiber substrate 1 has reinforcing fibers 3and 4 in the longitudinal direction and in the transverse directionintersecting it at a right angle and the second fiber substrate 2 hasreinforcing fibers 5 and 6 in bias directions relative to the abovedirections.

An example shown in FIG. 5 is a reinforcing fiber material wherein thenumber of laminations of the above-mentioned first and second fibersubstrates 1 and 2 are increased. In this fiber material, a first fibersubstrate 1 having longitudinal and transverse reinforcing fibers 3 and4 are a second fiber substrate 2 having reinforcing fibers 5 and 6 inthe bias directions are laminated alternately one by one to form sevenlayers and these layers are integrated by stitch yarns 7 passingupwardly and downwardly through the layers.

Also in this case, quasiisotropy can be obtained by lamination of thefirst fiber substrate 1 and the second fiber substrate 2 and besides astill thicker reinforcing fiber material can be obtained. In this case,the first fiber substrate 1 and the second fiber substrate 2 can belaminated alternately one by one as in the embodiment, but the first andsecond fiber substrates can be laminated alternately so as to formgroups consisting of one to several substrates. Further, as shown inFIG. 6, a structure can be possible in which the laminated substrates 1and 2 are slided in its longitudinal direction so that the laminate hasa varied thickness.

Furthermore, it is not necessarily required that the first fibersubstrate 1 and/or the second fiber substrate 2, by themselves, havesimultaneously reinforcing fibers 3, 4, 5 and 6 crossing in twodirections. That is, when these fiber substrates 1 and 2 have only onegroup of reinforcing fibers 3 or 4, and 5 or 6, they are satisfactory ifthey in the state of a laminate of a plurality of layers arequasiisotropy as a whole. FIG. 7 shows an embodiment for such case.

The reinforcing fiber material shown in FIG. 7 is constructed bylaminating a plurality of the first fiber substrates 1a and 1b, thesecond fiber substrate 2a and the third fiber substrate 2b andintegrating these substrates by stitch yarns 7. The first fibersubstrate 1a is a unidirectional prepreg prepared by gathering byimpregnating longitudinally arranged reinforcing fibers 3 with anuncured thermosetting resin, the first fiber substrate 1b is aunidirectional prepreg prepared by gathering by impregnatingtransversely arranged reinforcing fibers 4 with an uncured thermosettingresin, the second fiber substrate 2a is a unidirectional prepregprepared by gathering by impregnating reinforcing fibers 5 in the biasdirection with an uncured thermosetting resin, and the third fibersubstrate 2b is a unidirectional prepreg prepared by gathering byimpregnating reinforcing fibers 6 in the bias direction with an uncuredthermosetting resin. The bias angle with respect to the longitudinaldirection of the second fiber substrate 2a is +(25-65) degree, and thebias angle with respect to the longitudinal direction of the third fibersubstrate 2b is -(25-65) degree. In this way, the reinforcing fibermaterial formed by laminating the fiber substrates has, as a whole,reinforcing fibers 3, 4, 5 and 6 in respective directions and possessesquasiisotropy within the surface.

For the individual fiber substrate, a unidirectional woven fabric can beused instead of the above-mentioned prepregs consisting only ofreinforcing fibers. That is, it is possible to use a so-calledunidirectional woven fabric in which the reinforcing fibers are usedonly in one direction as warp or weft and these reinforcing fibers aresupported by auxiliary fibers as weft or warp. Of course, a woven fabricprepreg impregnated with a resin can also be used.

In the reinforcing fiber material shown in FIG. 8, a pair of first fibersubstrates 1 having the above-mentioned reinforcing fibers 3 and 4 isarranged inside, a pair of second fiber substrates 2 having thereinforcing fibers 5 and 6 is arranged outside and these substrates areintegrated by stitch yarns 7. Here, binding by the stitch yarns 7 isperformed so that the four layers are integrated only in the centralregion and in both side regions B, B, the four layers are separated atan intermediate layer. However, two adjacent first and second fibermaterials 1 and 2 may be integrated in both side regions B, B. Thisstructure is advantageous for molding FRP beams hereinafter described.

The reinforcing fiber material shown in each of the above-mentionedembodiments can be in the form of a prepreg prepared by preimpregnationwith an uncured resin or in the state not impregnated with a resin. Inthe latter case, at the time of molding into FRP a resin is intrudedinto a metal mold while the fiber material is charged in the mold.

In the above-mentioned embodiments, in case where the fiber substrates 1and 2 consist of woven fabrics, the woven fabrics have a plain weave.However, other weaves such as satin weave and twill weave can also beused. Furthermore, the so-called noncrimp fibrous structure prepared byintegrating, by auxiliary fibers, two yarn groups prepared by gatheringbend-free straight reinforcing fibers in parallel to one another in theform resembling a sheet, as disclosed in U.S. Pat. No. 4,320,160 ispreferred because the reinforcing fibers do not have bends on which astress is concentrated, its characteristic property can be fullyexhibited and the mechanical strength and breaking strength of FRP canbecome still higher. Furthermore, satin weave is preferred because it ispossible to obtain a thicker woven fabric substrate (per sheet) ascompared with other weaves, the resistance of yarn slippage isrelatively small and, during needling stitch yarns by a needle, thereinforcing fibers of the fiber substrates are hardly damaged.Furthermore, with respect to the second fiber substrate 2 in which thereinforcing fibers are bias, the weave slide arising when a tension isapplied in the prepreg step or the molding step can be prevented byprevious passing of stitch yarns along the longitudinal direction.Accordingly, it is suitable to produce fiber reinforced plastics havinguniform physical properties.

As the reinforcing fibers which are used in this invention, there arepreferably used multifilament yarns such as carbon fiber, glass fiber,polyaramide fiber, silicon carbide fiber or metal fiber. Of these,carbon fiber is the most suitable. This carbon fiber includes, needlessto say, a so-called graphite fiber having a graphitize structure.

Moreover, when the reinforcing fibers are made into a woven fabric, thereinforcing fibers include glass fiber, polyaramide fiber, siliconcarbide fiber, alone or in combination of at least two types. It is mostsuitable that the twist number to be given to reinforcing fibers used asmultifilament yarns is substantially zero and at most 20 T/m. When thetwist number becomes too large, the resin impregnation during theproduction of FRP becomes poor and it is difficult to heighten thevolume content of the fiber as a composite material. Furthermore, whenreinforcing fibers for stitching are passed through by a needle, thereinforcing fibers are readily broken by the needle. This tendencybecomes marked particularly in case of carbon fiber which has anextremely high modulus of elasticity and a low elongation at break andis brittle to bend.

It is preferred that the carbon fiber which is used in this inventionhas an elongation at break of 1 to 2.2%, and particularly, 1.7 to 2.2%as measured by a method specified in JIS C 7601. Moreover, it ispreferred that the monofilament diameter is 4 to 12μ. It is alsopreferred that the carbon fiber used as reinforcing fibers has across-sectional area of 0.07 to 3.5 mm². If the cross-sectional area ofreinforcing fibers is smaller than 0.07 mm², only a thin fiber substratecan be obtained. Accordingly, to obtain fiber material having aconsiderable thickness, lamination of more fiber substrates is necessaryand if this is done, the FRP is to have more interlayer parts, that is,weak points. Moreover, such lamination is not preferred from theviewpoint of production because the number of lamination stepsincreases. On the contrary, when the cross-sectional area is larger than3.5 mm², it becomes difficult to infiltrate the resin uniformly into theinterior because of excessive thickness. Furthermore, when thesubstrates are woven fabrics, bend by crossing among the reinforcingfibers becomes excessively large, concentrating a stress on this bendportion, and accordingly it becomes impossible to fully utilize thecharacteristic properties of the reinforcing fibers.

It is preferred that the stitch yarns 7 have an elongation at breaklarger than those of the reinforcing fibers 3, 4, 5 and 6. That is, asmentioned below, because the stitch yarns have large bend at upper andlower surfaces of the fiber materials, an extreme stress concentrationoccurs to the stitch yarns when a stress is given to FRP. Accordingly,it becomes possible to prevent break at the stitch yarns by selecting anelongation at break of the yarns larger than those of the reinforcingfibers.

The stitch yarns are preferably selected from the group consisting ofmultifilament yarns of carbon fiber, glass fiber, polyaramide fiber,silicon carbide fiber and metal fiber. If the reinforcing fibers arecarbon fibers, the stitch yarns are preferably selected from the groupconsisting of glass fiber and polyaramide fiber. The monofilamentdiameter of glass fiber used as stitch yarns is preferably 3 to 7μ andthat of polyaramide fiber is preferably 5 to 20μ.

To prevent delamination by stitch yarns effectively, it is preferred topass the stitch yarns through a plurality of laminated substrates at anangle substantially perpendicular to the surface of the fibersubstrates. Also in case where the stitch yarns are biased relative tothe perpendicular of the surface of the fiber substrate, it is preferredthat the bias angle is in the range: -15 to 15 degree, and preferably,-5 to 5 degree with respect to its perpendicular. Generally, FRP hascharacteristics that it exhibits a high strength along its fiber axisdirection but the strength decreases sharply as the angle to the fiberaxis direction becomes larger. That is, it is preferred for theeffective prevention of delamination that the stitch yarns are passedthrough in the state where the yarns exhibit their highest strength.When the stitch yarns are passed through an angle outside the abovecritical angle, more stitch yarns are required to prevent thedelamination. This is not preferable because the possibility of thefiber substrate to be damaged by a needle is increased.

It is preferred that these stitch yarns have characteristics that theyarns have a thermal shrinkage at 120° C. of not higher than 2%,preferably, a thermal shrinkage at 180° C. of not higher than 1%. Thisis because, as mentioned above, the function of stitch yarns consists inprevention of delamination of fiber substrates and when heat is applied,for example, during the drying step of prepreg production, a stress isgiven by an extreme thermal shrinkage of the stitch yarns and a break atthe stitch portion becomes apt to occur.

With respect to the thickness of the stitch yarns, it is preferred thatthe cross-sectional area is 0.01 to 0.25 mm². When the thickness is toolarge, the stitch yarns protrude beyond both surfaces of the laminatedfiber substrates and when molded into FRP, it is impossible to heightenthe volume fraction of the reinforcing fiber. On the contrary, when thethickness is too small, more stitch yarns are required to prevent thedelamination and accordingly the number of needle passages is increasedand the possibility of the fiber damage is increased particularly incase where carbon fibers are used as reinforcing fibers.

It is preferred that the stitch yarns are composed of multifilamentyarns and that in this case the reinforcing fibers have the lowestpossible twist number, preferably, a twist number of 30 to 70 T/m inorder to heighten the volume fraction of the reinforcing fibers in theproduction of FRP. Moreover, with respect to seaming with stitch yarns,single chain stitching in which only one yarn entangles with itself toform loops or lock stitching in which two yarns, i.e., upper yarn andlower yarn, entangle with each other can be used. Preferably, singlechain stitching is used.

It is preferred that both the stitch length of stitch yarns and theinterval between adjacent stitch yarns are 2 to 30 mm. When the stitchlength and the stitch interval are smaller than 2 mm, the number ofstitches, i.e., the number of needle passages necessary to pass thestitch yarns, increases and the possibility of the reinforcing fibers tobe damaged increases unfavorably. When they exceed 30 mm, the stitchdensity becomes too large to obtain a sufficient stitch effect.

With respect to the passing length of stitch yarns, it is preferred tobe 0.9 to 1.1 times the thickness of an FRP molding and more suitablysubstantially equal to it. When the passing length of stitch yarns issmaller than 0.9 time the thickness of an FRP molding, the fibersubstrates are fastened too strongly and infiltration of resin into thesubstrates, the reinforcing fibers and the monofilaments becomesdifficult. Furthermore, when molding is performed by a mold, aspindle-shaped clearance is formed between the stitch portion and theinner surface of the mold, and this clearance is filled with resin. Onthe other hand, when the passing length is longer than 1.1 times, anexcessive yarn length corresponding to passing length of stitch yarnminus thickness of FRP molding becomes too long, and the stitch yarnsare bent in the direction of the thickness of the fiber material and thestitch effect becomes difficult to obtain.

With respect to the resin for the impregnation of reinforcing fibermaterial, it is preferably a resin selected from the group consisting ofan unsaturated polyester resin, an epoxy resin, a phenol resin and apolyimide resin, for both impregnation to prepare prepregs and intrusionof resin during molding into metal mold.

In case where fiber substrates are composed mainly of woven fabrics, theresin content based on the reinforcing fiber material is preferably 35to 60% by volume. When the resin content is lower than 35 % by volume inthe molding of FRP, the amount of resin is not sufficient to fill thegeometrical space confined by the fiber substrates and the reinforcingfibers, voids increase, bondability between the fiber substrates and thereinforcing fibers decrease and the FRP properties, particularly tensilestrength and shear strength, deteriorate. On the contrary, when theresin content is higher than 60 % by volume, the content of reinforcingfibers becomes too low. Since characteristic properties of FRP, such asstrength and elastic modulus, vary in approximate proportion to thecontent of the reinforcing fibers, with a resin content exceeding 60 %by volume, the above characteristic properties deteriorate because ofextremely low content of the reinforcing fibers. A preferable content ofresin is 40 to 55 % by volume.

Furthermore, the resin content in case where the fiber substrates areunidirectional prepregs is preferably 30 to 60% by volume for the samereason as above. Preferably, the content is 35 to 50 % by volume. Here,the reason why the lower limit of the resin content is somewhat lowerthan the above case is that the geometrical space confined by the fibersubstrates and the reinforcing fibers is somewhat smaller in the case ofunidirectional prepregs. A problem in case where the reinforcing fibersconsist of carbon fiber is that it is weak to bend and brittle. Owing tothese characteristic properties partial break of carbon fibers is apt tooccur when the stitch yarns are passed by a needle. The partial break ofcarbon fibers, when the fibers are made into a composite material,inevitably results in a decrease in strength of the resulting FRP. Inorder to make stitching by stitch yarns possible by eliminating thepartial break of carbon fibers, it is preferred that the fiber substratecomposed of carbon fibers has a resistance to yarn slippage of a wovenfabric of 40 to 600 g/cm, preferably 100 to 500 g/cm. If it exceeds 600g/cm, the partial break of carbon fibers by a needle becomes marked,while if it is lower than 40 g/cm, the fiber yarns are apt to zigzagduring the formation of the woven fabric and the form of the wovenfabric becomes unstable.

Here, the weave slide resistance of a woven fabric means a resistancemeasured according to JIS L 1079-1966 method. That is, as shown in FIG.11A, three test pieces (5 cm along warp and 15 cm along weft) are cutfrom a fiber substrate. A transverse yarn situated at a 0.5 cm interval(b) contiguous to a 2.0 cm interval (a) is removed, a comb-shaped pinspecified in JIS is inserted into the portion (b), the test piece is seton a tensile testing machine, the portion (c) is pulled at a constantrate of 10 cm/min, the maximum pulling resistance of the yarn at theportion (a) is measured. The data are expressed by an average of threemeasurements (rounded to an integer). Also, in case where the crossingreinforcing fibers are not in an angular relation of 90 degree, testpieces cut along warp and weft as shown in FIG. 11B are prepared, andthe test pieces are measured according to the above-mentioned method.

When the above-mentioned weave slide resistance of a woven fabric issatisfied, it becomes possible even in case of a fiber substrate usingcarbon fiber which is fragile to bend as reinforcing fibers to stitch upby stitch yarns without partial break of the carbon fibers.

With respect to a needle for stitch yarn application, one having theleast possible cross-sectional area, a sharp point and a smooth surfaceis preferred. The use of a needle having a surface provided with zigzagbarbs, a so-called felt needle should be avoided because such a needlecauses partial break of carbon fibers.

The reinforcing fiber material according to this invention is placed ina metal mold after it has been converted into a prepreg by impregnationwith an uncured resin or it is placed in a metal mold and impregnatedwith a resin. Then, by heat curing, an excellent thick FRP havingquasiisotropic properties and being free from delamination can beobtained.

The reinforcing fiber materials of this invention are particularlyuseful for FRP plates which must have quasiisotropy toward the interiorof the sheet and can be substituted for duralumin, which is aconventional light metal alloy. For example, the materials are usefulfor skin materials of an airplane wing or H- or I-beams.

FIG. 9 illustrates an FRP I-beam molded on the basis of the technicalidea of the reinforcing fiber materials according to this invention. InFIG. 9, an FRP I-beam 10 has a reinforcing fiber material shaped into abeam having an I cross-section and having a web 13 and flanges 14. Toconstruct a reinforcing fiber material, it has four sheets of plainweave fabrics 102A, 102B, 102C and 102D of carbon fibers, correspondingto the above-described second fiber substrates. These four fabrics 102A,102B, 102C and 102D are laminated at the portion of the web 13 so thatwarps 15 and wefts 16 are situated at angles of 45 degree and -45degree, respectively, relative to the longitudinal direction of the beam10, and integrated by stitching by stitch yarns 17 composed of glassfiber multifilament yarns. On the other hand, said fabrics 102A, 102B,102C and 102D are diverged in the flange portions 14 in the oppositedirections to form separate sets of the plain weave fabric 102A and102B, and 102C and 102D. On each of the diverged surface are laminatedtwo sheets of plain weave fabrics 101 of carbon fiber, corresponding tothe above-described first fiber substrates so that their warps 11 andwefts 12 have angles of 0 and 90 degrees, respectively, relative to thelongitudinal direction of the beam 10, and are integrated with saidplain weave fabrics 102A and 102B, and 102C and 102D by stitch yarns 17composed of glass fiber multifilaments and extending in the longitudinaldirection of the beam 10. Each of the spaces confined by the plain weavefabrics 102A and 102B, and 102C and 102D, and the plain weave 101 isfilled with an adjusting material consisting of carbon fibers in adirection selected so that its longitudinal direction coincides with thelongitudinal direction of the beam 10.

A resin 21 is infiltrated into the reinforcing fiber material thusconstructed and heat-cured. Of course, in addition to plain weave, theabove-mentioned fabrics of satin weave, twill weave or noncrimp fibrousstructure can be used as the fiber substrates.

Preferably, the number of lamination of fiber substrates is at least 2.By properly selecting this number and portions to be stitched ordiverged, beams having a variety of shapes, as shown in FIGS. 10Athrough 10I, can be obtained in addition to the I-beam shown in thisembodiment. Furthermore, different types of sheet-like substrates can belaminated, for example, in an alternating manner. Further, when thesubstrates consist of woven fabrics, they can be laminated so that thedirections of the warp and weft of the fabric differ, for example, inthe beam of the embodiment, the yarn direction on the outermost layer is0 degree relative to the longitudinal direction and the yarn directionon the two inner layers is 45 degree. A layer in which the warp and weftdirections are ±45 degree relative to the longitudinal direction of thebeam can react effectively with a shearing stress produced when alongitudinal bending stress is given to the beam.

The fiber substrates need be stitched at the web portion of the beam,but not at the flange portions. Furthermore, it is also possible to forma flange by diverging the substrates to form sets of plain weave fabric102A and 102B, and 102C and 102D without laminating at the flangeportion, stitching or not stitching each set of the plain weave fabrics.However, the lamination of a plain weave fabric 101, i.e., fibersubstrate, on the diverged surface as in FIG. 9 is preferred because itis possible to give the same thickness to the web and the flanges.

The above-described beam has high mechanical strength, particularly,interlayer shear strength and delamination strength, because there arestitch yarns also in the direction of lamination of a plurality ofsubstrates. Moreover, the beam has an increased breaking strengthbecause, when a crack forms on the lamination surface, the propagationof this crack is prevented by the stitch yarns in the direction oflamination.

As has been described, the reinforcing fiber materials of this inventioncan be given quasiisotropic properties by laminating a plurality offiber substrates having different directions of reinforcing fibers, andcan give delamination-free, high-reliability materials for reinforcingplastics by integrating the laminate of the substrates with stitch yarnspassing through in the direction of lamination.

EXAMPLE 1

A plain weave fabric, warp and weft densities 5 ends/cm and 5pitches/cm, respectively, in which the reinforcing fibers extended intwo directions including the longitudinal direction and a directionintersecting therewith at a right angle (hereinafter referred to aswoven fabric A) was obtained by using, as the reinforcing fibers, carbonfibers "TORAYCA" T-300 (a product of Toray Industries, Inc., averagemonofilament diameter of 7μ, filament number of 3,000, cross-sectionalarea of 0.11 mm², twist number of 15 T/m, strand strength of 330 kg/mm²,strand modulus of 23.5×10³ kg/mm², elongation at break of 1.4%).

On the other hand, a bias woven fabric in which the reinforcing fibersextended in two directions having angles of ±45 degree relative to thelongitudinal direction (hereinafter referred to as woven fabric B) wasobtained by using the above "TORAYCA" T-300, making a double weavefabric of plain weave having warp and weft densities of 5 ends/cm and 5pitches/cm, respectively, and cutting and opening this hollow orcircular weave fabric helically at an angle of 45 degree relative to thelongitudinal direction.

Then, said woven fabrics A and B were laminated alternately one by onein the same longitudinal direction to form a 10-layer laminate andsecured temporarily at its edge with a tape to obtain a quasiisotropiclaminate (hereinafter referred to as comparative laminate).

On the other hand, a quasiisotropic laminate was obtained from thecomparative laminate using, as stitch yarns, yarns prepared by giving atwist of 40 T/m to polyaramide fiber "KEVLAR⃡- 49 (average monofilamentdiameter of 12μ, 1420 denier, cross-sectional area of 0.11 mm²,elongation at break of 2.4%; a product of E.I. du Pont Co., Ltd.) bysingle thread chain stitching along the warp direction of the wovenfabric A by needling under conditions including a stitch length of 10 mmand stitch interval of 10 mm. (This laminate was hereinafter referred toas laminate of this invention).

Then, the above comparative laminate was cut along the warp and weftdirections of the woven fabric A to form a plate (35 cm×35 cm). Thisplate-form composite laminate was set within a metal mold (inside size35 cm×35 cm) whose periphery was sealed with silicone rubber, the metalmold was preheated to about 80° C., while it was evacuated by a vacuumpump, and an epoxy resin having a viscosity lowered to about 2 poise byheating to 80° C. was intruded into the mold at a pressure of 3 kg/cm²to infiltrate the resin into the comparative laminate. Thereafter, thetemperature was raised to 180° C. and the resin was cured by holding itin this state for about 2 hours to obtain an FRP plate having athickness of 2.2 mm and a reinforcing fiber of 47.1 % by volume fraction(hereinafter referred to as comparative FRP). The above epoxy resinconsisted of a mixture of 100 parts by weight of "EPIKOTE" 827, an epoxyresin made by Petrochemicals Co., 90 parts by weight of Methyl NadicAnhydride (NMA) made by the same company and 1 part by weight of2-ethyl-4-methylimidazole (EMI) made by Shikoku Kasei Co., Ltd. Further,an FRP plate using the laminate of this invention and having a thicknessof 2.2 mm, a reinforcing fiber of 46.5% by volume fraction and a stitchyarn of 1.4% by volume fraction (hereinafter referred to as FRP of thisinvention) was obtained in quite the same manner as above.

Then, the above comparative FRP was cut in a direction in which thelongitudinal direction coincided with the warp direction of the wovenfabric A to form a test piece having a length of 210 mm and a width of20 mm. This test piece was subjected to a tensile test by using atensile testing machine, IS-5000, made by SHIMADZU CORP. underconditions including a tension rate of 5 mm/min to measure a tensilebreak load and a tensile modulus of elasticity. The tensile break loadwas 1850 kg and the tensile modulus of elasticity was 4,750 kg/mm². Onthe other hand, with respect to the FRP of this invention, a quitesimilar test piece was made and subjected to a similar test. The tensilebreak load was 2,085 kg and the tensile modulus of elasticity was 4,700kg/mm².

Thus, contrary to the anticipation that, since in the FRP of thisinvention, the stitch yarns are bent greatly because of single chainstitching and form loops on one surface of the plate, a stress isconcentrated on the bent portion and a break develops from the portionin the early stage and that owing to damage to the reinforcing fibers bya needle during passing of stitches, the tensile strength becomes low,the tensile break load of the FRP of the present invention was higher by235 kg (about 13%) though its reinforcing fiber content was lower thanthat of the comparative FRP, and besides its tensile modulus wassomewhat lower but differs little. Accordingly, it was recognized thatthe integrating effect of stitch yarns was excellent.

EXAMPLE 2

In quite the same manner as in EXAMPLE 1 except that the total number oflaminations of woven fabrics A and B was 26, there were obtained a6.1-mm thick FRP using the comparative laminate (reinforcing fiberfraction of about 48.8% by volume, hereinafter referred to ascomparative FRP) and an FRP plate using the laminate of this inventionand having the same thickness (reinforcing fiber fraction of about 48.9%by volume, stitch yarn fraction of about 0.6% by volume, hereinafterreferred to as FRP of this invention). These plates were cut so that thelongitudinal direction coincided with the warp direction of the wovenfabric A to form test pieces having a length of 152 mm and a width of102 mm.

Then, each of the above test pieces was subjected to a test forlongitudinal compression by using a universal testing machine (capacity30 ton) made by SHIMADZU CORP. The compressive strength of thecomparative FRP was 44 kg/mm², whereas that of the FRP of this inventionwas 49 kg/mm² and was higher by about 10%. This can be attributed to thefact that, in the FRP of this invention, the stitch yarns restrain athermal strain due to the difference in coefficient of linear thermalexpansion between the reinforcing fibers and the resin, a Poisson'sstrain difference resulting from the fact that the respective layershave different Poisson's ratios and a deformation of the plate in thedirection of the thickness resulting from compression.

Furthermore, in order to check an influence of an impact uponcompressive strength, a falling weight impact having an energy of 275kg.cm was applied to the center of each of the above test pieces in thedirection vertical to the surface and the test pieces were subjectedsimilarly to the test. As a result, the compressive strength of thecomparative FRP was 22 kg/mm², which was lower than that before theimpact by 50%, whereas that of the FRP of this invention was 30 kg/mm²and the drop was as low as about 30%. This can be attributed to the factthat, in the FRP of this invention, the stitch yarns preventedpropagation of the cracks produced by the impact.

EXAMPLE 3

In quite the same manner as in EXAMPLE 1, there were obtained acomparative FRP using the comparative laminate and an FRP of thisinvention using the laminate of this invention. In each test piece, apolytetrafluoroethylene (PTFE) film, length 35 cm, width 5 cm, thickness100μ, having no adhesion to the resin was inserted, before molding, intoan edge portion between the fifth fabric and the sixth fabric to givefactitious initial delamination to the edge of the plate. With respectto the FRP of this invention, stitch yarns had been removed from thepart into which the film was inserted. That is, integration by stitchyarns was applied to the 35×30 cm part only.

Next, each FRP was cut so that the film-inserted portion was situated atthe vertex of an isosceles triangle having a base length of 104 mm andan oblique line of 300 mm. Then, test pieces were prepared by bondingaluminum plates having the same shape and a thickness of 10 mm to bothsurfaces of the test pieces with an epoxy type adhesive. A pin wasinserted into each of the aluminum plate of the vertex of each testpiece.

Next, the relationship between the resistance to delamination and themoving distance of pin was determined by pulling the pin of each testpiece in the vertical direction of the surface of the plate at a speedof 2.5 mm/min by use of the above-mentioned tensile testing machine. Theresults of this measurement are shown in FIG. 12. As can be seen fromFIG. 12, in the comparative FRP, new delamination continuous to theinitial delamination was developed between the fifth and sixth layerfabrics at a relative low resistance as low as 30 kg (point A). Thisdelamination was suppressed by the bonding strength of the resin forsome time, but propagated at point B at a stretch and the resistancebecame almost zero.

On the contrary, in the FRP of this invention, new delamination wasproduced first at point C, but its resistance was about 76 kg, which was2.5 times higher than that of the comparative FRP. Furthermore, thepropagation of the delamination was suppressed by the stitch portionand, unlike the comparative FRP, the delamination did not propagate at astretch. When the stitch yarns were broken, new delamination wasproduced again, but similarly to the above, it was suppressed by thenext stitch portion and did not propagate at a stretch.

EXAMPLE 4

The FRP of this invention in EXAMPLE 3 to which no aluminum plate wasattached was subjected to delamination by pulling the vertex in thedirection vertical to the plate. Then, when the portion of broken stitchyarns was observed from the surface of the plate (from the side oppositeto the surface of delamination), "KEVLAR"- 49 used as the stitch yarnsappeared clear yellow, and the color difference was observed between theportion of broken stitch yarns and the portion of unbroken stitch yarns.Such a phenomenon can be attributed to the fact that, in the portion ofbroken stitch yarns, fine cracks are formed in the resin present in theportion or surroundings and the diffused reflection of light occurs.That is, because of presence of the stitch yarns, a stress isconcentrated on this portion and cracks are produced in a localizedmanner (if there were no stitch yarns, the cracks would occur throughoutthe surface). This indicates a possibility that, when the reinforcingfiber material of this invention is used, the faults developed withinthe interior can be found from a color change of stitch yarns observedfrom the exterior even when a CFPP which is black and whose interior isdifficult to observe is used.

What is claimed is:
 1. A fiber material for reinforcing plastics, which comprises:(1) a plurality of first woven fabrics comprised of multifilament yarns of carbon fibers having a twist number of at most 20 T/m and a cross-sectional area of 0.07 to 3.5 mm² which extend in two directions including the longitudinal direction and the transverse direction intersecting therewith at a substantially right angle, (2) a plurality of second woven fabrics comprised of multifilament yarns of carbon fibers having a twist number of at most 20 T/m and a cross-sectional area of 0.07 to 3.5 mm² which extend in two directions including directions having angles of (25-65) degrees and -(25-65) degrees relative to the longitudinal direction, wherein said first and second woven fabrics, each in the form of a set consisting of one to several woven fabrics, are laminated alternately with each other in the same longitudinal direction, and (3) continuous stitch yarns integrating all the above laminated first and second woven fabrics in a single chain stitching along the longitudinal direction of the fiber material, in a manner of repeatedly passing through the woven fabrics in the direction of lamination thereof with a passing length of 0.9 to 1.1 times the thickness of the fiber material for a reinforcing plastics, wherein said continuous stitch yarns are multifilament yarns composed of reinforcing fibers selected from the class consisting of glass fibers and polyaramide fibers having a cross-sectional area of 0.01 to 0.25 mm², an elongation at break greater than that of said carbon fibers in the woven fabrics and a thermal shrinkage at 120 degrees C. of not higher than 2%.
 2. A fiber material as set forth in claim 1, wherein the first and second woven substrates are woven fabrics having a weave selected from the group consisting of noncrimp fibrous structure, plain weave, twill weave and satin weave.
 3. A fiber material as set forth in claim 1, wherein the weave slide resistance of the woven fabric ranges from 40 to 600 g/cm.
 4. A fiber material as set forth in claim 3, wherein the yarn slippage resistance of the woven fabric is 100 to 500 g/cm.
 5. A fiber material as set forth in claim 1, wherein the woven fabric is impregnated with an uncured resin to form a prepreg.
 6. A fiber material as set forth in claim 5, wherein the resin is selected from the group consisting of an unsaturated polyester resin, an epoxy resin, a phenol resin and a polyimide resin.
 7. A fiber material as set forth in claim 5, wherein the resin is 35 to 60% by volume fraction.
 8. A fiber material as set forth in claim 7, wherein the resin is 40 to 55% by volume fraction.
 9. A fiber material as set forth in claim 1, wherein the reinforcing fibers comprising the woven fabrics have substantially no twist.
 10. A fiber material as set forth in claim 1, wherein the reinforcing fibers comprising the woven fabrics are multifilament yarns of carbon fiber and the stitch yarns are multifilament yarns selected from the group consisting of glass fiber and polyaramide fiber.
 11. A fiber material as set forth in claim 1, wherein the direction of the stitch yarns has an angle of -15 to 15 degree relative to the direction of lamination of the woven fabrics.
 12. A fiber material as set forth in claim 11, wherein the direction of stitch yarns has an angle of -5 to 5 degree relative to the direction of lamination of the woven fabrics.
 13. A fiber material as set forth in claim 11, wherein the direction of stitch yarns has an angle of substantially zero degree relative to the direction of lamination of the woven fabrics.
 14. A fiber material as set forth in claim 1, wherein both the stitch length of stitch yarns and the interval between adjacent stitch yarns are 2 to 30 mm.
 15. A fiber material as set forth in claim 1, wherein the passing length of stitch yarns is substantially the same as the thickness of a fiber reinforced plastic molding.
 16. A fiber material as set forth in claim 1, wherein the stitch density of stitch yarns is varied within the surface.
 17. A fiber material as set forth in claim 1, wherein the stitch yarn are reinforcing fibers having a thermal shrinkage at 180 degrees C. of not higher than 1%. 